It is known that block distortion that is perceived in a shape of pixel blocks occurs in a decoded image obtained by decoding the pixel blocks encoded in units of blocks. Such block distortion deteriorates a subjective image quality. Deblocking filters have been presented as a method for reducing such distortion.
Deblocking filters have been introduced into the moving image compression encoding standards such as H.264/AVC (ITU-T H.264|ISO/IEC 14496-10 Advanced Video Coding) and H.265/HEVC (ITU-T H.265|ISO/IEC 23008-2 High Efficiency Video Coding). Deblocking filters used in these standards perform filtering to change the pixel values for the respective pixels adjacent to the boundary of the pixel block. Because image decoding apparatuses equipped with such deblocking filters also perform filtering on a pixel block that is positioned on the upper side of the pixel block being processed, output delay occurs.
A method is known as another image decoding apparatus. In the method, pixel values are referred to in the deblocking filter, and the pixels on which processing is performed are reduced. The image decoding apparatus can reduce the scale of the memory circuit and simplify the processing, but requires an independent memory for referring to pixels necessary for filtering. In addition, the image decoding apparatus still causes the output delay described above.